1. Field of the Invention
The present invention relates to an apparatus for detecting carbon monoxide included in a hydrogen-containing reactant gas, an apparatus for detecting an organic compound included in the reactant gas, and an apparatus for detecting a lower alcohol included in the reactant gas. The present invention also pertains to the respective methods of detecting carbon monoxide, an organic compound, and a lower alcohol.
2. Description of the Related Art
Fuel cells are known apparatus in which the chemical energy of a fuel is converted directly into electrical energy. The fuel cell generally has a pair of electrodes arranged across an electrolyte, where the surface of one electrode is exposed to reactive gaseous hydrogen or gaseous fuel while the surface of the other electrode being exposed to an oxidizing gas containing oxygen. The electrical energy is generated between the electrodes through the electrochemical reactions occurring by the exposure.
A gaseous fuel supplied to such fuel cells is generated by a reformer, where methanol is steam-reformed according to the following reactions: EQU CH.sub.3 OH.fwdarw.CO+2H.sub.2 -21.7 kcal/mol (endothermic reaction)(1) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 +9.8 kcal/mol (exothermic reaction)(2 ) EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2 -11.9 kcal/mol (endothermic reaction) (3)
Carbon monoxide (CO) generated through the reaction expressed by Equation (1) is converted to carbon dioxide (CO2) by the reaction of Equation (2), and thus does not participate in the reforming reaction expressed by Equation (3). The rate of reaction expressed by Equation (1) may be different from the rate of reaction expressed by Equation (2), depending upon the reaction conditions, such as temperature and pressure. Carbon monoxide (CO) generated in the reaction of Equation (1) thus remains and is adsorbed by platinum catalyst or platinum-containing alloy catalyst on the fuel electrode and interferes with the catalytic action of platinum. This is generally referred to as poisoning of catalyst. Generators utilizing such fuel cells accordingly require a structure allowing the presence of carbon monoxide in the gaseous fuel fed from the reformer.
A variety of carbon monoxide sensors have been developed for determining the concentration of carbon monoxide included in a supply of gaseous fuel fed to the fuel cells.
A typical example is a potentiostatic electrolysis-based carbon monoxide sensor, which utilizes potentiostatic electrolysis applied to electrochemical analysis in solutions. FIG. 21 schematically illustrates a conventional potentiostatic electrolysis-based carbon monoxide sensor, which includes three electrodes; that is, a reference electrode P1, a counter electrode P2, and a working electrode P3.
These electrodes P1, P2, and P3 are exposed to a phase of electrolytic solution and a gas phase. When carbon monoxide comes into contact with the working electrode P3, an anode reaction shown below proceeds: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +2H.sup.+ +2e.sup.-
while a cathode reaction shown below proceeds on the counter electrode P2 exposed to oxygen included in the air: EQU (1/2)O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
The total reaction is accordingly expressed by: EQU CO+(1/2)O.sub.2 .fwdarw.CO.sub.2
The concentration of carbon monoxide is then determined by measuring the electric current generated through the oxidation of carbon monoxide with an ammeter P4.
This conventional carbon monoxide sensor can determine the concentration of carbon monoxide included in the air with high precision, but is significantly affected by the presence of hydrogen because of its principles of measurement. The conventional potentiostatic electrolysis-based sensor is thus not suitable for determining the concentration of carbon monoxide included in the hydrogen-rich gaseous fuel, which contains only a trace amount of carbon monoxide in an extremely large amount of hydrogen. The sensor naturally has a low sensitivity of detection to hydrogen, which is approximately 1/80 that to carbon monoxide. Since a supply of gaseous fuel fed to fuel cells contains an extremely large amount of hydrogen, the sensor simultaneously detects hydrogen and carbon monoxide and suffers from a problem of low precision in the measurement of carbon monoxide.
Like the potentiostatic electrolysis-based sensor described above, another known carbon monoxide sensor based on catalytic combustion has been developed originally for measuring carbon monoxide included in the air. The problem of poor precision thus arises in the process of measuring carbon monoxide included in the hydrogen-rich gaseous fuel.
Apparatus for detecting organic compounds and those for detecting lower alcohols have also been proposed and utilized.
Known methanol-detecting apparatus, which correspond to both the organic compound-detecting apparatus and the lower alcohol-detecting apparatus, are used for detecting methanol included in gasoline (for example, JAPANESE PATENT PUBLICATION GAZETTE No. H-3-48533). Such an apparatus is constructed as a cell including an ion-exchange membrane and two electrodes arranged across the ion-exchange membrane, where gasoline is fed to one electrode and an electrolytic solution, that is, 10 percent by weight of aqueous sulfuric acid, to the other electrode. On the electrode exposed to gasoline, a reaction expressed by Equation (4) below proceeds to generate carbon dioxide, hydrogen ions and electrons from methanol and water included in gasoline. On the electrode exposed to the electrolytic solution, a reaction expressed by Equation (5) below proceeds to generate water from hydrogen ions permeated through the ion-exchange membrane and oxygen and electrons included in the electrolytic solution. EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.- ( 4) EQU (3/2)O.sub.2 +6H.sup.+ +6e.sup.- .fwdarw.3H.sub.2 O (5)
Electromotive force generated between the electrodes by these electrochemical reactions increases with an increase in concentration of methanol included in gasoline. This apparatus accordingly determines the concentration of methanol included in gasoline, based on the electromotive force generated between the electrodes.
These conventional methanol-detecting apparatus, which are used for detecting methanol included in gasoline, that is, a liquid, can not measure methanol included in a gas, especially, a hydrogen-rich gas. Precise measurement of methanol included in a hydrogen-rich gas leads to efficient operation of methanol reformers for generating a hydrogen-rich gas through the reaction of methanol with water or to efficient operation of fuel cells or other mechanisms for generating electrical energy using as a fuel the hydrogen-rich gas generated by the methanol reformer.
This problem is not characteristic of the methanol-detecting apparatus, but similar problems are also found in apparatus for detecting lower alcohols other than methanol or those for detecting other organic compounds, when petroleum, instead of methanol, is used as material of the reformer.